Existing systems for directly injecting fuel into the combustion chamber of an internal combustion chamber switch between a first operating mode and a second operating mode.
A distinction is made in this context between a so-called stratified mode as a first operating mode, and a so-called homogeneous mode as a second operating mode. Stratified mode is used in particular at lesser loads, while homogeneous mode is utilized when greater loads are applied to the internal combustion engine.
In stratified mode, fuel is injected into the combustion chamber during the compression phase of the internal combustion engine, in such a way that at the moment of ignition, a fuel cloud is located in the immediate vicinity of a spark plug. This injection event can be accomplished in different ways. For example, it is possible for the injected fuel cloud already to be present at the spark plug during or immediately after the injection event, and to be ignited by it. It is also possible for the injected fuel cloud to be guided to the spark plug by a charging movement, and only then ignited. With both combustion methods, what is present is not a uniform fuel distribution but rather a stratified charge.
The advantage of the stratified mode is that in it, the lesser loads that are being applied can be handled by the internal combustion engine with a very small quantity of fuel. Greater loads, however, cannot be handled with the stratified mode.
In the homogeneous mode provided for such greater loads, fuel is injected during the intake phase of the internal combustion engine, so that the fuel can readily experience turbulence and thus be distributed in the combustion chamber. In this respect, homogeneous mode corresponds approximately to the method of operation of internal combustion engines in which fuel is injected in conventional fashion into the intake duct. If necessary, homogeneous mode can also be used at lesser loads.
In stratified mode, the throttle valve in the intake duct leading to the combustion chamber is opened wide, and combustion is controlled in open-loop and/or closed-loop fashion only by way of the fuel mass that is to be injected. In homogeneous mode, the throttle valve is opened or closed as a function of the requested torque, and the fuel mass to be injected is controlled in open-loop and/or closed-loop fashion as a function of the aspirated air mass.
In both operating modes, i.e., in stratified mode and in homogeneous mode, the fuel mass to be injected is additionally controlled in open-loop and/or closed-loop fashion, by a plurality of further operating variables, to a value that is optimum in terms of fuel economy, exhaust emissions reduction, and the like. The open-loop and/or closed-loop control is different in the two operating modes.
It is necessary to switch the internal combustion engine over from stratified mode to homogeneous mode and back again. Whereas in stratified mode the throttle valve is opened wide and air is thus supplied largely unthrottled, in homogeneous mode the throttle valve is only partially open and thus reduces the supply of air. Especially when switching over from stratified mode into homogeneous mode, the ability of the intake duct leading to the combustion chamber to store air must be taken into consideration. If it is not taken into account, the switchover can result in an increase in the torque delivered by the internal combustion engine.